ATLAS B5.1 test is a 1% small break loss of coolant accident (SBLOCA) at the top of the reactor pressure vessel with a total failure of a high-pressure injection (HPI) system. It was conducted as a counterpart test of LSTF SB-PV-07. The LSTF is scaled from a three-loop pressurized water reactor (PWR) Westinghouse design, while the ATLAS facility is scaled from APR1400. Thus, to determine B5.1 test conditions, a scaling analysis has been conducted between the two test facilities of LSTF and ATLAS. We reviewed the scaling method used for the ATLAS facility and found the initial and boundary condition for the counterpart test. Then, we compared the experimental data of the ATLAS B5.1 test against that of LSTF SB-PV-07. To reduce the scaling distortion between two IET facilities, we modified the RELAP5 input of the ATLAS facility toward the LSTF facility considering the elevation difference and design difference. The internal design difference between the ALTAS and LSTF facilities generated a significant difference in the plant transient beyond the scaling distortion.